What it means
Microsoft personal-account aliases are email addresses or phone numbers associated with one account. They share a password, contacts, subscriptions and storage. Sign-in preferences let the owner decide which aliases can be used as usernames to sign in.
A real-world example
An old public Outlook address receives many attempted logins. You add or keep another sign-in alias and block the old one from sign-in, while still receiving messages sent to it. You have not made a second account.
What to do
Check the allowed-alias list directly at account.microsoft.com and test an allowed sign-in before leaving the current session. Keep MFA and a unique password.
The distinction that matters
Disabling sign-in is different from removing an alias or changing the primary address. Email sent to the address can still arrive, so a scam sent to that mailbox still needs ordinary caution. Microsoft's support page describes the shared account data and the per-alias sign-in switch.
Microsoft's alias documentation
